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Oldest human genome reveals roots of first Americans

Siberia was a genetic melting pot of Europeans and Asians before they crossed over to America

(Image: Maria Stenzel/National Geographic/Getty)

By Michael Marshall

A 24,000-year-old boy from Siberia is helping redraw the Native American family tree. He is the oldest human to have his genome sequenced, and the results suggest that the first people to colonise the Americas were not simply east Asians. Instead, those early settlers had both western Eurasian and east Asian roots.

The first Americans probably arrived from north-east Asia via a land bridge, around 15,000 years ago. Confusion arose because Native Americans have DNA that resembles that of people in east Asia, but no modern contemporary population exactly fits, says Eske Willerslev of the Natural History Museum of Denmark in Copenhagen.

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Straight out of Siberia

Trying to figure out what happened, Willerslev and colleagues sequenced the genome of a boy who lived in southern Siberia 24,000 years ago. He is the oldest modern human to be sequenced, although other hominins and unrelated animals have been sequenced from far further back in time – often by Willerslev’s group.

The team found that the boy’s Y chromosome resembled those of modern western Europeans and all his chromosomes had a lot in common with those of Native Americans, and appeared to be ancestral to them. But there were no signs that he had east Asian relatives. A second Siberian skeleton, from 17,000 years ago, produced similar results.

The simplest explanation is that Native Americans are the result of two populations that interbred, says Willerslev. One ultimately gave rise to modern east Asians, while the other, which included the Siberian boy, gave rise to today’s western Europeans. “Native Americans are not just a group of east Asians,” says Willerslev.

Missing link

Nobody had expected that, but it explains a lot, says David Reich of Harvard Medical School in Boston, Massachusetts. “This feels like a missing link that pulls together the observations. It’s a very important paper.”

For instance, there is now no reason why Native Americans should match any particular contemporary east Asian population, because they are not solely descended from east Asians.

What’s more, the reason Native Americans carry some “European” DNA is that some of their ancestors were also the ancestors of modern Europeans.

Last year Reich showed that modern Europeans are a heavily mixed population, some of whose ancestors were most closely related to Native Americans. Willerslev’s findings explain that&colon; the same ancient Eurasian population contributed genes to modern Europeans and to Native Americans.

Clearly, there was a lot of back-and-forth between Europe and Asia during the last ice age. “The steppe region of Eurasia was a corridor for people to move,” says Reich, allowing migrations along a line from modern-day Hungary all the way to the Pacific. He says the interbreeding probably took place in Siberia rather than after the groups had crossed to the Americas.